Gibson et al, U.S. Pat. No. 3,957,583 discloses a process and apparatus for conducting antibiotic susceptibility tests in a relatively short time. In U.S. Pat. No. 3,957,583, the clinical specimen is examined directly without isolating the suspected microorganisms. The basic process of U.S. Pat. No. 3,957,583 involves introducing a specimen into blends of a selective culture medium and known antimicrobial agents. If the specimen contains a microorganism which is favored by the culture medium of a blend, and the microorganism is not susceptible to the agent, the optical characteristics of the blend will change.
When the concentration of organisms reaches 10.sup.5 in a well, the mechanism disclosed in U.S. Pat. No. 3,957,583 will indicate a positive reading and then a 100 to 1 dilution is made when the organism is inoculated into the media contained in the wells of this invention. The wells are covered with a tape which allows a limited and controlled amount of oxygen to flow into the microenvironment of the well.
The major problem encountered with conventional microbial sensitivity tests is the length of time that passes from obtaining a clinical specimen from the patient to selection of a suitable antimicrobial agent that can be expected to control any microorganisms detected. U.S. Pat. No. 4,000,041 discloses a medium for detecting E. Coli microorganisms. However, that patent falls short of disclosing how to determine the susceptibility of E. Coli microorganisms to a given set of antibiotics. Accordingly, it is an object of the present invention to provide a medium which, when used with the apparatus described in U.S. Pat. No. 3,957,583, will allow a clinician to determine within a relatively short period of time (from about 8 to about 12 hours) which of a series of antimicrobial agents will be effective in suppressing the activity of infectious microorganisms.
One other problem is present in the present application that is not a problem in conventional antibiotic sensitivity compositions. Namely, it is critical that the composition of the present application be adhered to strictly because of the microenvironment in which it is placed. The apparatus of U.S. Pat. No. 3,957,583 uses wells which accept only about 17 microns of dried medium and there is less than one microliter of antibiotic in the well. With the amounts of ingredients at this micro level, the intermolecular or electrostatic action between the minute quantities of ingredients in each well may affect the ability of the ingredients to react on the same level and in exactly the same manner as when using larger amounts. Antibiotics are particularly susceptible to this problem. The close confines of the small well also restrict the activity of the organism and the reactivity of chemicals and antibiotics are not predictable in this environment.
Another matter which is significant in the apparatus of U.S. Pat. No. 3,957,583 is that the flow of oxygen to the ingredients in the well must be controlled. In other words, the wells in which the medium is placed have a controlled oxygen (O.sub.2) environment and for this purpose, certain chemicals are included in the various media. Also as mentioned, the tape covering each well has a controlled and limited oxygen permeability.
It is also important to prevent release of chloride ions in the rehydrated medium to prevent the medium from becoming acid and giving a false positive as the metabolism of E. Coli itself produces an acidic change in the medium which change is shown by the change in color of the indicator. All of these problems are so much more significant when one is using a microsystem and make the standard full scale methodology inapplicable and not transferable to the present application.
The broth of the present invention will allow the clinician to test several antibiotics simultaneously to determine E. Coli sensitivity. The results obtained allow the clinician to select a primary antibiotic and also allow him to select a group of backup antibiotics, should the primary antibiotic fail to treat the patient's condition.